1. Field of the invention
The present invention relates, in general, to a dielectric material for high frequencies and, more particularly, to a dielectric material for high frequencies which possesses high quality factor Q and exhibits superior temperature coefficient of resonant frequency even in high frequency regions.
2. Description of the Prior Art
In recent years, there have been rapidly developed communication systems using microwaves (frequency band ranging from 300 MHz to 300 GHz), such as wireless telephones, car phones, cellular phones, satellite broadcasting systems, and the like, and there is an increasing demand the dielectric ceramics with better electrical properties, which are extensively used in resonator devices, band pass filters, microwave integrated circuits and other parts of communication systems.
For application for communication systems using microwaves, a dielectric material for high frequencies needs to satisfy the following conditions:
1. A large dielectric constant for the miniaturization of parts of communication systems because the wave length of the microwaves in a dielectric material is inversely proportional to the square root of the dielectric constant; PA0 2. A high Q value (reciprocal of dielectric loss) for high performance because dielectric loss is proportional to frequency; PA0 3. A small temperature coefficient of resonant frequency, so as to obtain desired resonant characteristics which are stable to temperature change. PA0 i. A Ba(M.sup.+2.sub.1/3 M.sup.+5.sub.2/3)O.sub.3 system wherein M.sup.+2 .dbd.Mg or Zn, M.sup.+5 .dbd.Ta or Nb [reference: K. Matsumoto, T. Hiuga, K. Takada and H. Ichimura, "Ba(Mg.sub.1//3 Ta.sub.2/3)O.sub.3 Ceramics with Ultra-low Loss at Microwave Frequencies" In Proce. of the Sixth IEEE International Symposium on Applications of Ferroelectrics, pp. 118.about.121, (1986)]. PA0 ii. A Ba.sub.2 Ti.sub.9 O.sub.20 system [reference: S. Nisikaki et al., "BaO--TiO.sub.2 --WO.sub.3 Microwave Ceramics and Crystalline BaWO.sub.4 " J. Am. Ceram. Soc., 71(1), C-11-C-17 (1988)]. PA0 iii. A (Zr,Sn)TiO.sub.4 system [reference: K. Wakino et al., "Microwave Characteristics of (Zr, Sn)TiO.sub.4 and BaO--PbO--Nd.sub.2 O.sub.3 --TiO.sub.2 Dielectric Resonators" J. Am. Ceram. Soc. 67(4), 278.about.281 (1983)]. PA0 i. A BaO--Sm.sub.2 O.sub.3 --TiO.sub.2 system [reference: J. M. Wu and M. C. Chang, "Reaction Sequence and Effects of Calcination and Sintering on Microwave Properties of (Ba,Sr)O--Sm.sub.2 O.sub.3 --TiO.sub.2 Ceramics" J. Am. Ceram. Soc., 73(6), 1599.about.1605 (1990)]. PA0 ii. A (Ba,Pb)O--Nd.sub.2 O.sub.3 --TiO.sub.2 system [reference: K. Wakino et al., "Microwave Characteristics of (Zr,Sn)TiO.sub.4 and BaO--PbO--Nd.sub.2 O.sub.3 --TiO.sub.2 Dielectric Resonators" J. Am. Ceram. Soc. 67(4), 278.about.281 (1983)]. PA0 iii. A (Pb,Ca)ZrO.sub.3 system [reference: J. Kato, "Material Produces Small Resonators with High Dielectric Constant" JEE, Sep., 114.about.118 (1991)].
In addition, it is required that the dielectric material for high frequencies is resistant to ageing, large in thermal conductivity and high in mechanical strength.
Well known representative dielectric materials for high frequencies will be exemplified.
On the one hand, the following are known as dielectric materials which are low in dielectric loss but have a dielectric constant of not more than 40:
On the other hand, the following are known as dielectric materials which have a dielectric constant of not less than 80 yet are relatively high in dielectric loss (Q.times.fo (GHz)&lt;10,000):
Dielectric ceramics for high frequencies having high dielectric constants are suitable as materials for microwave devices using electric waves of long wavelength and are in great demand in devices for communication systems requiring miniaturization.
However, it is very difficult to develop dielectric materials having stable temperature coefficients of resonant frequency as well as high dielectric constants and high Q values. Generally, dielectric losses and temperature coefficients of resonant frequency in dielectric materials having large dielectric constants, both increase because of dipole coupling therein. That is to say, since materials with high dielectric constants are inclined to have low Q values and large temperature coefficients of resonant frequency, it is very difficult to satisfy the three desired electrical properties in a single material system.
For application in communication systems, however, dielectric materials for high frequencies, first of all, must have stable temperature coefficients of resonant frequency.
Of the known conventional dielectric compositions, in fact, a material satisfying the three desired electrical properties at same time has not been found. For example, a SrTiO.sub.3 system exhibits satisfactory dielectric constant and quality factor (Q.times.fo) which are in a range of 110 to 151 and 8,900 to 36,000 GHz, respectively but is problematic in that the temperature coefficient of resonant frequency is too large, ranging from 505 to 7,500 ppm/.degree. C. On the other hand, a (Li.sub.1/2 Nd.sub.1/2)TiO.sub.3 system exhibits a low dielectric constant of about 75, and a temperature coefficient of resonant frequency of about -274 ppm/.degree. C. yet a small dielectric loss.